This invention relates to plasma doping systems used for ion implantation of workpieces and, more particularly, to methods and apparatus for eliminating the displacement current from current measurements in plasma processing systems.
Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. In a conventional ion implantation system, a desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Exacting requirements are placed on semiconductor fabrication processes involving ion implantation with respect to the cumulative ion dose implanted into the wafer, implant depth, dose uniformity across the wafer surface, surface damage and undesirable contamination. The implanted dose and depth determine the electrical activity of the implanted region, while dose uniformity is required to ensure that all devices on the semiconductor wafer have operating characteristics within specified limits. Excessive surface damage, particularly chemical etch, or contamination of the surface can destroy previously fabricated structures on the wafer.
In some applications, it is necessary to form shallow junctions in a semiconductor wafer, where the impurity material is confined to a region near the surface of the wafer. In these applications, the high-energy acceleration and the related beam forming hardware of conventional ion implanters are unnecessary. Accordingly, it has been proposed to use Plasma Doping (PLAD) systems for forming shallow junctions in semiconductor wafers.
In a plasma doping system, a semiconductor wafer is placed on a conductive platen located in a chamber, and the platen functions as a cathode. An ionizable gas containing the desired dopant material is introduced into the chamber, and a high voltage pulse is applied between the platen and the anode (or the chamber walls) causing the formation of a plasma having a plasma sheath in the vicinity of the wafer. The applied voltage causes ions in the plasma to cross the plasma sheath and to be implanted into the wafer. The depth of implantation is related to the voltage applied between the wafer and the anode. A plasma doping system is described in U.S. Pat. No. 5,354,381 issued Oct. 11, 1994 to Sheng.
In the plasma doping system described above, the high voltage pulse generates the plasma and accelerates positive ions from the plasma toward the wafer. In other types of plasma implantation systems, known as Plasma-Source Ion Implantation, PSII, systems, a separate plasma source is used to provide a continuous plasma. (These implantation systems are also known by several other acronyms, the most common being Plasma-Immersion Ion Implantation, PIII) In such systems, the platen and the wafer are immersed in this continuous plasma and, at intervals, a high voltage pulse is applied between the platen and the anode, causing positive ions in the plasma to be accelerated toward the wafer. Such a system is described in U.S. Pat. No. 4,764,394, issued Aug. 16, 1988 to Conrad.
Both PLAD and PSII systems require accurate dose measurement to achieve high-quality semiconductor devices. One approach to dose measurement in plasma doping systems involves measurement of the current delivered to the workpiece by the high voltage pulses, as described in the aforementioned U.S. Pat. No. 5,354,381. However, this approach is subject to inaccuracies. The measured current includes electrons generated during ion implantation and excludes neutral molecules that are implanted into the workpiece, even though these neutral molecules contribute to the total dose. Furthermore, since the measured current passes through the wafer being implanted, it is dependent on the characteristics of the wafer, which may produce errors in the measured current. Those characteristics include emissivity, local charging, gas emission from photoresist on the wafer, etc. Thus, different wafers may give different measured currents for the same ion dose.
A technique for dosimetry on a plasma based doping system is described by E. Jones et al. in IEEE Transactions on Plasma Science, Vol. 25, No. 1, February 1997, pp. 42-52. Measurements of implant current and implant voltage are used to determine an implant profile for a single implant pulse. The implant profile for a single pulse is used to project the final implant profile and total implanted dose. This approach is also subject to inaccuracies, due in part to the fact that it depends on power supply and gas control stability to ensure repeatability. Furthermore, the empirical approach is time consuming and expensive.
In comparison, typically in known beamline implantation systems, cumulative ion dose and uniformity are measured with Faraday cups, or a Faraday cage. The Faraday cage is typically a conductive enclosure, often with the wafer positioned at the downstream end of the enclosure and constituting part of the Faraday system. The ion beam passes through the Faraday cage to the wafer and produces an electrical current. The Faraday current is supplied to an electronic dose processor, which integrates the current with respect to time to determine the total ion dosage. The dose processor may be part of a feedback loop that is used to control the ion implanter.
Dose and dose uniformity have also been measured in conventional beamline ion implantation systems using a corner cup arrangement as disclosed in U.S. Pat. No. 4,751,393 issued Jun. 14, 1988 to Corey, Jr. et al. As discussed in the ""393 patent, a mask having a central opening is positioned in the path of the ion beam. The beam is scanned over the area of the mask with the portion passing through the central opening impinging on the wafer. Small Faraday cups located at the edge of the mask measure the beam current at these locations.
There is a need for accurate measurements of the ion current being delivered to a target in a PLAD or PSII system. The ion current must be properly measured, so that proper process conditions can be maintained. Regardless of the method of measuring current applied to the target, the measured current pulse includes capacitive, or displacement, current components that introduce errors in the measurement. This displacement current is the current observed when a capacitor, or any structure with capacitance, is subject to a rapid increase/decrease in the applied voltage. This current is required to provide the charge that maintains the relation V=Q/C, where V is the instantaneous applied voltage, C is the capacitance and Q is the instantaneous charge stored in the capacitor. In either a PLAD or PSII system, the cathode and anode together can be considered to be a capacitor C that is charged and discharged by the applied pulse. A Faraday cup, or similar current measurement tool, also has a capacitance and the measured current includes a displacement current. The current measurement signals generated by a Faraday cup, or similar measurement tool, are much smaller than other current measurements, since they represent only a portion of the current the workpiece is receiving. The capacitance of the Faraday cup is also smaller than the capacitance of the anode and the cathode, but not in the same proportion as the measured currents. As a result, the displacement current is a larger proportion of the total current measured by the Faraday cup. Also, since the signal-to-noise ratio is lower for Faraday cup measurements, the presence of displacement currents is less desirable. Thus, it is more important that the displacement current in Faraday cup measurements be removed.
Conventionally, the displacement current at the leading and trailing edges of the pulse is ignored. Unfortunately, in many applications, the voltage pulse lengths are in the range of twenty xcexcs (microseconds). The typical rise time of the pulse is about three xcexcs while the fall time is about seven xcexcs. Since the displacement current is observed during the rise and fall times, it contributes to ion current measurement errors over a significant proportion of the voltage pulse.
The displacement current contributes to an inaccurate measurement of ion current in a plasma processing system. Accurate measurement of the ion current is necessary to efficiently operate the system. Therefore, a way to reduce or eliminate the errors in ion current measurements which are caused by displacement current is needed.
According to a first aspect of the invention, a method of measuring an ion current in a plasma processing system including first and second electrodes, wherein a voltage pulse is applied across the first and second electrodes is provided. The method comprises electrically coupling a capacitor between the first and second electrodes to receive the voltage pulse; measuring a first current supplied to the first and second electrodes during the voltage pulse and providing a first current signal representative of the first current; measuring a second current supplied to the capacitor during the voltage pulse and providing a second current signal representative of the second current; and subtracting the second current signal from the first current signal to provide an ion current signal representative of the ion current.
The capacitor may have a same or nearly the same capacitance as a first capacitance between the first and second electrodes. Alternately, the capacitor may have a capacitance different from a first capacitance between the first and second electrodes; and the method includes adjusting at least one of the first and second current signals to compensate for the difference in capacitance.
In another variation, a target is disposed on the first electrode and the step of providing the first current signal comprises placing an ion detecting device adjacent the target to generate the first current signal, selecting a capacitor having a capacitance different from a capacitance of the ion detecting device; and adjusting at least one of the first and second current signals to compensate for the difference between the two capacitances. Alternately, the method comprises selecting a capacitor having a capacitance substantially the same as a capacitance of the ion detecting device.
According to another aspect of the present invention, an apparatus for measuring ion current in a plasma processing system wherein a voltage pulse is applied across first and second electrodes is provided. The apparatus comprises a capacitive device coupled to the first and second electrodes to receive the voltage pulse; first means for measuring a first current supplied to the first and second electrodes during the voltage pulse and for providing a first current signal representative of the first current; second means for measuring a second current supplied to the capacitive device during the voltage pulse and for providing a second current signal representative of the second current; and means for subtracting the second current signal from the first current signal to provide a measurement of the ion current in the plasma processing system.
In another aspect of the present invention, a method of measuring an ion current in a plasma processing system including first and second electrodes in a processing chamber is provided. The method comprises measuring a pulsed current through the chamber and providing a pulsed current signal that is representative of the pulsed current; providing a compensation signal that is representative of a displacement current component of the pulsed current; and subtracting the compensation signal from the pulsed current signal to provide an ion current signal that is representative of ion current delivered to a target.
In a variation of this aspect, the step of providing a compensation signal comprises simulating the displacement current component of the pulsed current. Alternately, the step of providing a compensation signal comprises: providing a capacitor having a capacitance that is the same or nearly the same as a capacitance between the first and second electrodes, applying a pulsed voltage to the capacitor in synchronism with the pulsed current, and measuring a second current supplied to the capacitor in response to the pulsed voltage and providing a second current signal that constitutes the compensation signal.